First Lady Michelle Obama says she and the US president are "outraged and heartbroken" over the kidnapping of 276 schoolgirls in Nigeria.

Mrs Obama, taking the rare step of delivering her husband's weekly radio address, called the abduction an "unconscionable act".

The First Lady spoke as intelligence sources told Sky News they believe the girls are now being kept in at least four separate groups, complicating the search.

The sources said they believe they know where some of the girls are.

The burnt-out school where the girls were taken from last month

"Like millions of people across the globe, my husband and I are outraged and heartbroken over the kidnapping of more than 200 Nigerian girls from their school dormitory in the middle of the night," Mrs Obama said in the address.

"This unconscionable act was committed by a terrorist group determined to keep these girls from getting an education - grown men attempting to snuff out the aspirations of young girls."

She said this violence "was not an isolated incident ... it's a story we see every day as girls around the world risk their lives to pursue their ambitions".

"In these girls, Barack and I see our own daughters," Mrs Obama said, referring to Malia, 15, and Sasha, 12.

"We see their hopes, their dreams and we can only imagine the anguish their parents are feeling right now."

Video:UK Girls Hit Hard By Kidnapping

Barack Obama has sent a team to help the Nigerian government locate the girls, who were kidnapped from a boarding school in Chibok in the northern Borno state on April 14.

The Boko Haram militant group is behind the kidnappings.

British and American officials are using advanced eavesdropping equipment to scan the Sambisa forest where the schoolgirls are thought to be, Sky News understands.

Video:Anger In UK Over Nigeria Girls

Sky's Special Correspondent Alex Crawford, who is in the Nigerian capital, Abuja, said if the intelligence is true it would be "much more difficult to mount simultaneous raids" to rescue them.

"The whole thing is fraught with danger," she added.

Some reports said Nigerian security forces failed to respond to warnings about Boko Haram's planned abduction of the girls.

Video:Cameron: Kidnappings 'Pure Evil'

Amnesty International said it had verified reports from several credible sources who claimed the military was aware of the impending attack close to four hours before it took place.

The Nigerian government has rejected the findings as "unfounded".

The leader of Boko Haram, Abubakar Shekau, has threatened to sell the girls "on the market", amid reports some have already been trafficked to neighbouring Chad and Cameroon.

1/12

Gallery: Stars In Nigeria Kidnap Campaign

Talk show host Ellen DeGeneres is one famous face to express sympathy for the girls, who were snatched at gunpoint from their Nigerian school.

But Nigerian President Goodluck Jonathan said he believes the girls are still in the country.

The kidnappings have drawn condemnation from around the world and celebrities have launched a Twitter campaign under the hashtag #bringbackourgirls.